Laminar flow hoods are used, in particular in the electronics industry, when manufacturing wafers for very large scale integration (VLSI) circuits.
Such wafers are manufactured in a sequence of accurately determined operations of the photo-etching type, to form a complex repetitive pattern on the wafer. At the end of manufacture, the wafer is cut up in order to obtain as many integrated circuits as there are patterns on the wafer. Clearly a fault in the pattern of any of the integrated circuits obtained in this way makes that circuit unusable, and it is obviously advantageous to reduce the number of such defects in order to increase production yield.
It should be pointed out that the etched wafers comprise different kinds of circuit at different stages of manufacture. During a first stage, they comprise useful circuit devices which are not yet interconnected, while during a second stage they comprise devices which are interconnected to form genuine circuits which are sensitive to any electrical signals that may be applied thereto.
Laminar flow hoods are used firstly in order to protect the items being handled from the effect of dust: even very small quantities of dust can reduce production yield to a very great extent. Any circuit element having a particle of dust adhering thereto is unusable.
In order to prevent the air surrounding the wafer from bringing in dust, the feed air is filtered and the flow of this air is directed so that no polluted air returns towards the wafer from the floor environment or from the operators. Thus, a wafer placed on the working surface beneath a hood should be immersed solely in a laminar flow of filtered air. It is essential that the flow be laminar in order to avoid any turbulence in the air which could sweep in dust towards the working surface.
However, the problem is complicated by the fact that under the working conditions required in the VLSI electronic circuit industry, it is also necessary to use air having low humidity. Under such conditions, the various physical mechanisms capable of generating and accumulating static electricity on insulating surfaces become highly effective. In particular, the surface of the semiconductor wafers may take on a high electric charge. Such an electric charge sets up an electric field which attracts those rare particles of dust that remain in the laminar flow, in spite of the filtering performed at the inlet to the hood. This gives rise to the above-mentioned drawbacks relating to the manufacturing yield of VLSI circuits.
Further, once the wafers already have circuits on them, the appearance of electrostatic charge on the wafers can give rise to micro discharges which may destroy various semi-conductor devices.
Briefly, static electricity reduces the production yield of VLSI circuits for two reasons:
(a) prior to the circuits being interconnected, it increases dust pollution; and PA1 (b) once they have been interconnected, it destroys circuits by electrostatic discharge. PA1 provide strictly identical flows of positive ions and of negative ions in order to avoid progressively raising insulating surfaces or insulated conductors in the ionized flow to a high potential; PA1 avoid producing aerodynamic turbulence or any deformation in the laminar flow of the hood; PA1 avoid introducing water vapor, ozone pollution or pollution from microscopic particles, e.g. particles torn from the high voltage points under the effect of the electric field; PA1 avoid electrically charging the few remaining dust particles in the laminar flow; and PA1 avoid any danger of electrocuting personnel. PA1 a rigid frame which is insulating or which includes insulators for supporting wires; PA1 a first set of parallel conductive wires which are coplanar, equidistant and stretched over the rigid frame; PA1 a second set of parallel conductive wires which are coplanar, equidistant and stretched over the rigid frame, and each of which is at substantially equal distances between two wires of the first set; PA1 high tension insulating means between the wires of the first set and the wires of the second set; and PA1 an alternating high tension power supply, in particular having an output transformer whose output terminals are connected respectively to feed the wires of the first set and the wires of the second set; PA1 said high tension being selected to generate an alternating corona discharge in each gap between the wires of the second set, thereby producing flows of charges of both polarities which accompany the laminar flow and serve to eliminate static electricity inside the cabinet. PA1 the output from the amplifier is directly connected in series with the AC source; PA1 the output from the amplifier controls the cursor of a DC voltage divider; PA1 the output from the amplifier is connected via a resistance to the set of insulated wires, and the set of insulated wires is separated from the AC feed by a capacitor. PA1 the wires of the first set themselves; PA1 an auxiliary electrode placed in the working plane of the hood, with the wires of the first set being connected to ground; or else PA1 an auxiliary rigid frame having a conductive grid, as described below.
In order to improve production yield, it is therefore essential to get rid of the electric charge generated in a laminar flow hood by neutralizing it.
Two types of static electricity eliminator are suitable for being used in laminar flow hoods in general for solving this problem.
Firstly, radioactive eliminators may be used, comprising a pellet that emits alpha rays, for example. Such rays ionize air over a distance of a few centimeters from the pellet. By raising the pellet to a positive potential, for example, the negative ions produced are collected by the pellet and the remaining positive ions are available for neutralizing negative charge in the immediate environment of the pellet. By blowing on the ionized zone, a flow of air can be set up to convey ions towards the zones where charge is to be neutralized.
Such neutralizers have a low maximum discharge current (a few tens of pico-amps) and operate over a short distance (a few centimeters). They are therefore not very effective and they also suffer from the drawbacks related to the presence of a radioactive substance. As a result they are not usually employed in industrial laminar flow hoods.
In practice, industrial hoods employ static electricity eliminators using a corona discharge.
For fitting to laminar flow hoods, units comprising a grid of electrodes are used, with the electrodes generally being in the form of points facing bars and fed with alternating voltages, the entire assembly being associated with a fan setting up a flow of air through the grid, thereby entraining ions towards the surfaces to be neutralized. Such neutralizers include one made by the American company Techni-Tool, the Dynastat made by the American company CRP, or the Aerostat (registered trademark) made by the American company Simco. Such apparatuses, or components therefor, are also described in the following U.S. Pat. Nos.: 3,585,448, 4,092,543, 4,188,530, 4,216,518, and 4,423,462.
Their general principle is that during one full cycle of the feed voltage these units successively produce ions of both polarities. During the neutralization process, only the ions of one polarity are used and the others are eliminated by grounding.
Unlike radioactive devices, appropriately disposed eliminators based on corona discharge are capable of providing charge-neutralizing currents at several microamps suitable for neutralizing charged surfaces in a few tens of seconds and at a distance of greater than one meter. They need to satisfy the following conditions:
In the techniques currently in use, a sufficient ion density is generally obtained in the hood by using a flow of air coming from an auxiliary fan.
The positive and negative ion currents are equalized by adjusting the waveform of the alternating feed voltage, and examples are described in above-mentioned U.S. Pat. Nos. 2,879,395, 3,714,531, 4,092,543, and 4,423,462. Equal amplitude positive and negative voltage half-cycles do not provide equal positive and negative ion currents because positive and negative corona discharges do not have the same current/voltage characteristics.
It is difficult to avoid turbulence when using commercially available blowers. Strictly speaking, turbulence is satisfactory only with a purely static apparatus.
As to the problem of impurities, the water vapor question is generally solved by the air conditioning system of the hood. However, corona discharges do give off small quantities of ozone, and these are acceptable in principle provided that the alternating feed voltage does not exceed a critical threshold which is slightly greater than the breakdown threshold of the corona discharge.
In addition, the quantity of particles torn from the point-shaped parts of the device must be limited, i.e. erosion of the bare members of the electrostatic charge eliminator must be limited.
Thus, in addition to irremediably damaging the equipment and degrading its operation, arcing also produces erosion and consequently the emission of vapors and particles which poison the atmosphere in the hood. It is thus highly desirable to use a system which is designed so as to be incapable of arcing.
Finally, avoiding any danger of electrocution has been particularly closely studied by Simco, as described in U.S. Pat. Nos. 3,585,448 and 4,216,518.
However, in general, none of the devices known so far gives complete satisfaction.
In French Pat. No. 80 11945 (published under the number 2,483,259) and in the certificate of addition thereto number 81 09646 (published under the number 2,506,086), the present Applicant describes a particularly effective device for eliminating electrostatic charge, however it makes use of an auxiliary supersonic flow which is manifestly incompatible with the operating characteristics of a laminar flow hood.
It now appears that the problems encountered in practice with laminar flow hoods can be solved by using a new electrostatic charge eliminator which, in addition, avoids the drawbacks related to the auxiliary flow used by the Applicant's prior devices.